Se Precursor Research Articles

A simple synthesis of Ag2+x Se nanoparticles and their thin films for electronic device applications

A simple method to synthesize silver selenide nanoparticles has been proposed. By changing the ratio of Se-oleylamine complex and silver acetate in the reacting mixture at different temperatures, both size and stoichiometry of the silver selenide particles could be successfully controlled. The size of the nanoparticles was adjusted by changing reaction temperatures. The synthesized silver selenide nanoparticles showed size changes from 3 to 10 nm when the corresponding reaction temperatures were 40–100°C, respectively. In addition to the size change, the stoichiometry of the synthesized nanoparticles (Ag2+x Se) could be adjusted by simply varying the ratio of Ag to Se precursors. Through XPS analyses the x value in Ag2+x Se was determined, and it changed between 0.54 and −0.03 by varying Ag/Se ratio from 2/0.75 to 2/4. The optical property of the nonstoichiometric Ag2+x Se nanoparticles was different from that of stoichiometric Ag2Se nanoparticles, but showed the plasmon absorption of Ag-Ag network. The plasmon absorption was decreased with the increased concentration of the Se precursor. Finally, the Ag2+x Se thin film in this work showed large magnetoresistance and successfully applied to prepare high-performance Schottky diode. The Ag2.06Se film exhibited the magnetoresistance effect up to 0.9% at only 0.8 T at room temperature. The voltage drop and breakdown voltage of the Schottky diode were 0.5 V and 9.3 V, respectively.

Synthesis of hierarchical structure Cu2SnSe3 microsphere by a solvothermal method

Hierarchical structure ternary Cu2SnSe3 (CTSe) microsphere, consist of nanosheet constitutional unit, has been successfully synthesized by solvothermal route. The Se powder and ethylenediamine were chosen as the Se precursor and reaction solvent. Under 323K temperature and constant stirring condition, the Se powder slowly dissolved into the solvent with homogeneous dispersion. The solvent not only acted as reducing agent but also morphology control agent. The obtained CTSe microsphere represents 15–20μm size with the uniform secondary structure 20–30nm thickness through scan electron microscopy (SEM) measurement. The direct band gap was up to 1.6eV and the photocatalytic H2 generation rate was 1.9 times higher than Degussa P25 TiO2 under the simulated solar light illumination.

(Invited) Seebeck Coefficient Measurement of Pbsete / PbTe Nanolaminate Structures

Thermoelectric (TE) materials can be applied for solid state power generation or heating/cooling systems that convert heat into electricity and vice-versa. The green renewable technology related to thermoelectrics could play an important role to decrease the energy consumption from fossil fuels and decrease greenhouse gas emission. The main challenge for thermoelectric devices produced currently is their low efficiency. Therefore, it is essential to develop high efficiency TE materials and devices with superior performance. The efficiency of TE materials is expressed by the dimensionless thermoelectric figure of merit, ZT = S2σT/ k, where S is the Seebeck coefficient, σ is the electrical conductivity, and kis the thermal conductivity [1]. Previous work has demonstrated two main TE materials have higher ZT values. The first TE material with a complex crystal structure called a phonon glass electron crystal in which the voids, defects, and rattlers could scatter phonons to decrease the lattice thermal conductivity, and thus to increase ZT. Another one is called nanomaterials: including quantum wells, quantum dots, superlattice, quantum wires and nanolaminates, which introduce a large density of interfaces to enhance phonon scattering. The thermal conductivity of these nanomaterials was effectively reduced, and the ZT of TE materials was improved significantly. The IV-VI semiconducting lead chalcogenides, such as PbTe and PbSe, have been investigated due to their high figure of merit, good chemical stability, low vapor pressure and high melting point. Therefore, they are promising thermoelectric materials for intermediate temperature applications ranging from 600 to 800 K. There are different techniques that can be used to synthesis PbTe and PbSe, including pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering deposition, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). The ALD is a self-limiting technique that allows atomic layer growth each time. ALD can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interface. ALD also shows perfect conformal coverage when it is deposited on complex surface structures. Therefore, ALD is considered as a novel and competitive method to deposit PbTe/PbSe nanolaminate structures. It is possible to generate the reproducible and well-defined nanolaminate structures. In addition, the ALD deposition can be operated at rather low temperature compared to other techniques. In this work we report on the Seebeck coefficient measurements in PbSeTe / PbSe nanolaminate structures. The multiple PbTe/PbSeTe nanolaminates were deposited on silicon substrates by a thermal ALD system. This new nanolaminate structure was achieved by first depositing a very thin layer of the PbSeTe ternary compound with binary PbTe layers. The PbSeTe ternary compound is formed by nucleation of PbSe quantum dots between binary PbTe layers, and followed by an annealing process step. It is noted that the formation of the PbSe quantum dots is facilitated by the prevailing Volmer-Weber island growth mode. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2), (trimethylsilyl) telluride ((Me3Si)2Te) and (trimethylsilyl) selenide ((Me3Si)2Se) were employed as the chemical ALD precursors for lead, telluride and selenide, respectively. 20 sccm N2 was used as a carrier gas to transport the chemical precursors into the ALD reaction chamber. The ALD growth temperature was 170 oC. The solid lead precursor was volatilized at a temperature of 140 oC, the liquid Te precursor required heating to 40 oC, and the liquid Se precursor was kept at room temperature. The chamber base pressure was kept at 30 mTorr. Several physical characterization techniques have been employed to determine the ALD nanolaminate formation. The crystal structure was analyzed by X-ray diffraction (XRD). The film morphology and structure of the products were determined by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM).The surface roughness was analyzed by atomic force microscopy (AFM). The analysis of the composition and stoichiometry of the ternary and binary layers were carried out by X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray spectroscopy (EDS). The Seebeck coefficient of the sample was also measured in order to calculate ZT value using Seebeck coefficient measurement equipment of MMR SB100. Figure 1 shows an FE-SEM image of surface morphplogy of PbTe/PbSe nanolaminate films on silicon substrate.

Solvothermal synthesis of ternary CdSxSe1−x–graphene composites with improved photoelectric properties

CdSxSe1−x–graphene composites were prepared by a solvothermal process using cadmium acetate as Cd precursors, sulfourea as S precursors and selenium powder as Se precursors. CdSxSe1−x–graphene composites were characterized by X-ray diffraction, scanning electron microscope and ultraviolet–visible absorption spectroscopy. The photoelectric properties of CdSxSe1-x–graphene and bare CdSxSe1-x were investigated by transient photocurrent responses and three-electrode cyclic voltammetry. Compared with bare CdSxSe1−x, CdSxSe1−x–graphene composites show the improved photoelectric characteristics in the presence of graphene.

Crystallization Behavior of Solution-Processed CIGSe Thin Film Semiconductor by Stepwise Annealing Process.

CuIn(x)Ga1-xSe2 (CIGS) thin films were prepared by a solution-based CuInGa (CIG) precursor- selenization process. First, we investigated the effect of selenization temperature on the formation of polycrystalline CIGS and grain growth. The CIG precursor films were selenized using a two-step process to investigate the reaction of Se and CIG precursors during the formation of CIGS thin films. Depending on the temperature in the 1st step of the selenization process, the CIG precursor forms a different intermediate phase between the single phase to ternary phase such as Cu, Se, CuSe, InSe, and CuInSe2. In addition, the intermediate phase exerts a significant influence on the final phase obtained after the 2nd step of the selenization process, particularly with regard to characteristics such as polycrystalline structure and grain growth in the CIGS films. The photoelectron conversion efficiency of devices prepared using CIGS thin films was approximately 1.59-2.75%.

Catalytic synthesis of matchstick-like Ag2Se–ZnSe hetero-nanorods using Ag2S nanocrystals as seeds

In nanowire catalytic growth, the catalyst particles usually remain at the tip of as-grown nanowires, which can be utilized to prepare matchstick-like heterostructures at the nanoscale. Based on this feature and a solution-phase catalytic growth route, we herein report the synthesis of Ag2Se–ZnSe matchstick-like hetero-nanorods, consisting of Ag2Se head and ZnSe rod-like stem. Three different kinds of silver sources, including Ag(0), Ag2S, and Ag2Se, are selected as initial seeds for growing ZnSe crystalline nanowire/rods. By comparison with the case of Ag(0) or Ag2Se, the use of Ag2S nanoparticles, which alter the chemical composition of catalytic particles and convert to Ag2Se catalyst after adding Se precursor (SeO2), is highly effective for the formation of uniform Ag2Se–ZnSe hetero-nanorods. The reason for this result may be attributed to a synergistic effect between the size of catalyst particles and the chemical conversion of Ag2S to Ag2Se.

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

We have designed a new nonphosphinated reaction pathway, which includes synthesis of a new, highly reactive Se-bridged organic species (chalcogenide precursor), to produce bright white light-emitting ultrasmall CdSe nanocrystals of high quality under mild reaction conditions. The detailed characterization of structural properties of the selenium precursor through combined 77Se NMR and laser desorption ionization–mass spectrometry (LDI-MS) provided valuable insights into Se release and delineated the nanocrystal formation mechanism at the molecular level. The 1H NMR study showed that the rate of disappearance of Se precursor maintained a single-exponential decay with a rate constant of 2.3 × 10–4 s–1 at room temperature. Furthermore, the combination of LDI-MS and optical spectroscopy was used for the first time to deconvolute the formation mechanism of our bright white light-emitting nanocrystals, which demonstrated initial formation of a smaller key nanocrystal intermediate (CdSe)19. Application of thermal driving force for destabilization resulted in (CdSe)n nanocrystal generation with n = 29–36 through continuous dissolution and addition of monomer onto existing nanocrystals while maintaining a living-polymerization type growth mode. Importantly, our ultrasmall CdSe nanocrystals displayed an unprecedentedly large fluorescence quantum yield of ∼27% for this size regime (<2.0 nm diameter). These mixed oleylamine and cadmium benzoate ligand-coated CdSe nanocrystals showed a fluorescence lifetime of ∼90 ns, a significantly large value for such small nanocrystals, which was due to delocalization of the exciton wave function into the ligand monolayer. We believe our findings will be relevant to formation of other metal chalcogenide nanocrystals through expansion of the understanding and manipulation of surface ligand chemistry, which together will allow the preparation of “artificial solids” with high charge conductivity and mobility for advanced solid-state device applications.

Synthesis of Air-stable PbSe Quantum Dots Using PbCl2-oleylamine System

Air-stable monodisperse PbSe quantum dots were synthesized by a hot injection method with a simple PbCl2-oleylamine (PbCl2-OLA) system supplying both the Pb source and the capping ligand. Se powder dissolved in oleylamine (SeOLA) was used as the Se precursor. The sizes of the as-synthesized, monodisperse colloidal PbSe quantum dots ranged from 3.6nm to 13.9nm with a relative standard deviation of ∼2.8-8.4%. The air-stability of the as-synthesized PbSe quantum dots was confirmed by a systematic study with transmission electron microscopy. Both the air-exposed PbSe quantum dot monolayer film and the solution-stored PbSe quantum dots in n-hexane were air-stable for at least 24 months. This may provide a practical way to fabricate PbSe QDs based films for photovoltaic devices under ambient environment.

Phosphine-free synthesis of Ag-In-Se alloy nanocrystals with visible emissions.

As promising heavy metal-free emitting materials, Ag-In-Se nanocrystals (NCs) are conventionally synthesized using organic phosphine agents and exhibit near-infrared emissions. In this work, we demonstrate a rapid phosphine-free approach for synthesizing Ag-In-Se alloy NCs with the emissions tunable to the visible region on the basis of the phosphine-free dissolution of Se powder. At room temperature, Se powder is reduced by dodecanethiol and dissolved in oleylamine to produce a Se precursor. The resultant Se precursor is highly active, which permits rapid synthesis at a relatively low temperature, such as at 90 °C for 150 s. By optimizing the size, structure, and composition, the photoluminescence quantum yield of the as-synthesized Ag-In-Se NCs is enhanced to up to 10%. The growth of the Ag-In-Se NCs involves composition and phase transition, which strongly depend on the reaction temperature. The Ag2Se nuclei form first, and the Ag-In-Se NCs are produced by doping In(3+) into the preformed Ag2Se nuclei. Tetragonal phase Ag-In-Se is obtained below 170 °C, while the orthorhombic phase appears over 190 °C. The potential of Ag-In-Se NCs as red emitting phosphors for lighting-emitting diodes is further demonstrated.

Optical and cytotoxicity properties of water soluble type II CdTe/CdSe nanoparticles synthesised via a green method

ABSTRACTWe herein report the optical and cytotoxicity properties of highly luminescent water soluble mercaptopropanoic acid (MPA) capped CdTe/CdSe core shell nanoparticles (NPs). The synthesis of the CdTe/CdSe NPs was carried out via a simple, one pot and economical route, involving the use of greener materials under ambient environment in the absence of an inert atmosphere. The temporal evolution of the size and optical properties of the nanomaterials was investigated by varying the reaction time and stability of the as-synthesised material at pH 12. The as-synthesised nanomaterials were characterised using UV-vis absorption and photoluminescence (PL) spectroscopy. The nanoparticles obtained were of high quality with high absorption and emission features. Addition of Se precursor to produce CdSe layer on the CdTe NPs core surface resulted in significant red shirt of both the absorption and emission maxima. The stability study showed that the emission maximum peak positions and FWHM remain the same with increase in emission intensity for all the NPs during the aging period. The cytotoxicity assay showed very high cell viability for the CdTe/CdSe NPs produced at 7 h compared with those produced at 30 mins as the concentration increased from 0.1 to 60 ug/ml. The lower cytotoxicity at the higher reaction time was attributed to the higher stability of the material and hence lower release of Cd2+.

Effect of Binder Content in Cu–In–Se Precursor Ink on the Physical and Electrical Properties of Printed CuInSe2 Solar Cells

Printed chalcopyrite thin films have attracted considerable attention in recent years due to their potential in the high-throughput production of photovoltaic devices. To improve the homogeneity of printed CuInSe2 (CISe) layers, chemical additives such as binder can be added to the precursor ink. In this contribution, we investigate the influence of the dicyandiamide (DCDA) content, used as a binder in the precursor ink, on the physical and electrical properties of printed CISe solar cells. It is shown that the use of the binder leads to a dense absorber, composed of large CISe grains close to the surface, while the bulk of the layer consists of CISe crystallites embedded in a CuxS particle based matrix, resulting from the limited sintering of the precursor in this region. The expected additional carbon contamination of the CISe layer due to the addition of the binder appears to be limited, and the optical properties of the CISe layer are similar to the reference sample without additive. The electrical ch...

Pulse electrodeposition of copper selenides from acidic aqueous baths

Copper selenide compounds were synthesized at room temperature by a single-step electrodeposition process on Ti substrate from acidic aqueous solutions of Cu(NO3)2 and SeO2, employing constant and pulsed potential techniques. The electrochemical behavior of solutions with varying Cu(II)/Se(IV) precursor concentration ratio was investigated by linear sweep voltammetry. The combined effect of bath composition and deposition potential on the crystalline structure, atomic composition and morphology of the obtained deposits was determined. In contrast to potentiostatic deposition, the pulsed potential process allowed facile formation of crystallized copper selenides. Layers composed of a network microstructure of ca. 200nm-thick CuSe platelets, vertically oriented on the substrate, were produced.

Aberration Corrected STEM Study of the Surface of Lead Chalcogenide Nanoparticles

In this contribution, we report on the microscopic and spectroscopic characterization of lead chalcogenide nanoparticles. PbSe and PbTe nanoparticles were synthesized by chemical methods, using different molar ratios of Pb, Se, and Te precursors and varying the reaction time. These nanoparticles were characterized by means of transmission electron microscopy and their related techniques: electron diffraction, energy dispersive X-ray spectroscopy (EDXS), electron energy loss spectroscopy (EELS), and aberration corrected scanning transmission electron microscopy (AC-STEM), to obtain information related to their morphology, crystal structure, and composition. They were also characterized by means of Fourier transform infrared (FTIR) absorption spectroscopy to assess the functional groups found on the surface of the nanoparticles, depending on the synthesis conditions. The results obtained showed that the synthesized nanoparticles always display a higher concentration of Pb compared to that of the chalcogen. ...

ALD Synthesis of Ternary PbSeTe Layers Alternating with the Binary PbTe in Nanolaminate Structures for High Zt Thermoelectric Materials

Thermoelectric materials have attracted much attention for their potential in conversion between heat and electricity and consequently could be used for heating and cooling systems. The efficiency of a thermoelectric material is expressed by the dimensionless thermoelectric figure of merit, ZT = S2σT/ k, where S is the Seebeck coefficient, σ is the electrical conductivity, and kis the thermal conductivity [1]. Previous work demonstrated alloying was used to decrease the lattice thermal conductivity and thus to increase ZT owning to mass difference enhanced phonon scattering. In order to further progress in the development and commercialization of the next generation of thermoelectric devices it is necessary to enhance the ZT value. Most recently research focused on low dimensional nanostructures, including quantum wells, quantum dots, superlattice, quantum wires and nanolaminates to improve ZT. In superlattice structures, reduced lattice thermal conductivity is mainly due to the phonon scattering at multiple interface boundaries. Significant efforts have been made to investigate the IV-VI semiconducting lead chalcogenides, such as PbTe and PbSe due to their high figure of merit, good chemical stability, low vapor pressure and high melting point. Therefore they are promising thermoelectric materials for intermediate temperature applications ranging from 600 to 800 K.PbTe and PbSe films have been deposited by different techniques, such as pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Among these, ALD is considered as a novel and competitive method to deposit PbTe/PbSe nanolaminate structures. The ALD is a self-limiting atomic layer reaction in each ALD cycle introduce. It can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interface. ALD also can be used to deposit conformal films onto very complex structures. It is possible to generate the reproducible and well-defined nanolaminate structures. Furthermore, the deposition temperature of ALD is rather low in comparison with other growth techniques.In this work we report on the successful synthesis of multiple PbTe/PbSeTe nanolaminates on silicon substrates by a thermal ALD system. In this new nanolaminate structure a very thin layer of the PbSeTe ternary compound is formed by nucleation of PbSe quantum dots between binary PbTe layers, which is followed by an annealing process step. It is noted that the formation of the PbSe quantum dots is facilitated the prevailing Volmer-Weber island growth mode. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2), (trimethylsilyl) telluride ((Me3Si)2Te) and (trimethylsilyl) selenide ((Me3Si)2Se) were employed as the chemical ALD precursors for lead, telluride and selenide, respectively. 20 sccm N2 was used as a carrier gas to transport the chemical precursors into the ALD reaction chamber. The ALD growth temperature was 170 oC. The solid lead precursor was volatilized at a temperature of 140 oC, the liquid Te precursor required heating to 40 oC, and the liquid Se precursor was kept at room temperature. The chamber base pressure was kept at 30 mTorr.Several physical characterization techniques have been employed to determine the ALD nanolaminate formation. The crystal structure was analyzed by X-ray diffraction (XRD). The film morphology and structure of the products were determined by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM).The surface roughness was analyzed by atomic force microscopy (AFM). The analysis of the composition and stoichiometry of the ternary and binary layers were carried out by X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray spectroscopy (EDS).

Facile and Green Synthesis of CdSe Quantum Dots in Protein Matrix: Tuning of Morphology and Optical Properties

Herein, we have demonstrated a facile and green approach for the synthesis of Cadmium selenide (CdSe) quantum dots (QDs). The process was mediated by bovine serum albumin (BSA) and it was found that BSA plays the dual role of reducing agent as well as a stabilizing agent. The QDs exhibited sharp excitonic absorption features at ~500 nm and subsequently showed reasonably good photoluminescence (PL) at room temperature. The PL is seen to be strongly dependent on the concentration of the precursors and hence, the luminescence of these QDs could be conveniently tuned across the visible spectrum simply by varying molar ratio of the precursors. It can be envisaged from the fact that a red-shift of about 100 nm in the PL peak position was observed when the molar ratio of the precursors ([Cd2+]:[Se2-], in mM) was varied from 10:5 to 10:40. Subsequently, the charge carrier relaxation dynamics associated with the different molar ratio of precursors has been investigated and very interesting information regarding the energy level structures of these QDs were revealed. Most importantly, in conjunction with the optical tuning, the nanomorphology of these nanoparticles was found to vary with the change in molar ratios of Se and Cd precursors. This aspect can provide a new direction of controlling the shape of CdSe nanoparticles. The possible mechanism of the formation as well as for the shape variation of these nanoparticles with the molar ratios of precursors has been proposed, taking into account the role of amino acid residues (present in BSA). Moreover, the QDs were water soluble and possessed fairly good colloidal stability therefore, can have potential applications in catalysis and bio-labeling. On the whole, the present methodology of protein assisted synthesis is relatively new especially for semiconducting nanomaterials and may provide some unique and interesting aspects to control and fine tune the morphology vis-à-vis, their optical properties.

EFFECT OF OXYGEN ON SIZE-CONTROLLED SYNTHESIS OF CdSe QDs

Unique properties of quantum dots (QDs) are controlled by their customizable particle sizes which can be engineered to suit local need. One-pot organometallic injection synthesis of cadmium selenide (CdSe) QDs is reported. The operation started with the injection of a week old selenium (Se) precursor into a boiling non-coordinating octadecene (ODE) at 195oC resulting in the formation of monodispersed size tunable CdSe QDs with discrete homogeneous nucleation. Differences in the injection and withdrawal time of the sample resulted to the dissimilarity in the shape, size and the opto-electronic properties of the QDs. The effect of oxygen on the synthesized CdSe QDs was studied by exposing freshly prepared sample to atmospheric oxygen for 206 days. The samples were characterized using optical absorption spectroscopy (UV-vis), optical photoluminescence (PL), X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM), electron dispersive X-ray (EDX) X-ray diffraction spectroscopy (XRD), and X-ray photoelectron spectroscopy (XPS). Noticeable deformation on the size, shape and the crystalline orientation of the CdSe QDs were observed on the oxygen-interacted sample.Although the synthesis method is safe and produced good quality CdSe QDs, the interaction of the sample with oxygen degrades their opto-electronic quality.

Microstructure and phase evolution in single phase CuInSe2 particles synthesized using elemental precursors

This paper reports solid-state formation reaction pathways for CuInSe2 particles synthesized by ultrasonic irradiation of elemental precursors followed by thermal annealing. Effective heat of formation (EHF) model is used to predict the first formed and consecutively formed phases of Cu+Se, In+Se compounds. The XRD patterns showed that CuSe and In4Se3 phases are formed by ultrasonication of Cu+Se and In+Se precursors as predicted by the EHF model. As-sonicated Cu+In+Se precursors are thermally annealed and the CuInSe2 reaction is investigated based on the crystallographic relation between the binary phases. According to this model, when either CuSe2 or In4Se3 phases are involved in the CuInSe2 formation, the reaction is categorized under suppressed reactions due to the missing epitaxial relation. An activated reaction is reported between CuSe and InSe phases due to the epitaxial match. The experimental findings support the crystallographic model by the completion of CuInSe2 formation reaction at 350°C annealing temperature, which is after the phase transformation of CuSe2 to CuSe at 342°C.

Growth process and magnetic properties of α-FeSe nanostructures

Growth process and magnetic properties of PbO-type α-FexSe nanostructures with shape changing from nanocacti to nanopetals and then to nanosheets are investigated. With iron acetylacetonate [Fe(acac)3] and Se powder as raw materials, the diffusion process of Fe atoms dominates the synthesis of α-FexSe nanocacti following phase transitions from FeSe2 to Fe3Se4 and finally to α-FexSe. When a mixed solution containing Se precursor and Fe(acac)3 was used as the raw material, the formation of FeSe2 and Fe3Se4 can be avoided and, bended α-FexSe nanopetals can be prepared at 345 °C, which became flat nanosheets with a [001] preferred orientation as extending the reaction time from 1 to 4 h. No superconducting transition occurs in the α-FexSe (0.84 ≤ x ≤ 1.05) nanostructures due to composition heterogeneity or size effect. Magnetic measurements indicate that an antiferromagnetic component with a Néel point at about 45 K dominates the magnetic properties of the α-Fe0.87Se nanosheets.

Slow colloidal growth of PbSe nanocrystals for facile morphology and size control

We report colloidal growth of PbSe nanosheets and finely size-tuned PbSe nanocrystals (NCs) via simple control of reaction parameters. The approach involves slow injection of precursors with excess amounts of oleic acid. Retarded growth, due to both the slow supply of precursors and the surfeit of oleic acid, causes attachment of PbSe NCs through the (110) planes, which are more reactive than the (100) facet, into a two-dimensional geometry. In contrast, such attachment processes can be prevented by impurities, e.g., Cd chalcogenide (CdSe or CdS) NCs dispersed in chloroform. For instance, the slow injection of Pb and Se precursors into a reaction solution containing Cd chalcogenide NCs results in the growth of spherical PbSe NCs, as the Cd chalcogenide NCs hinder the PbSe nuclei from merging via (110) planes. Compared to conventional rapid-injection methods, PbSe NCs grow slowly, which enables fine control of NC size. Ab initio calculations suggest that Cd precursors strongly bound on the surface of PbSe NCs may impede nanosheet formation and may slow PbSe NC growth.

Cl-capped CdSe nanocrystals via in situ generation of chloride anions.

Halide ions cap and stabilize colloidal semiconductor nanocrystal (NC) surfaces allowing for NCs surface interactions that may improve the performance of NC thin film devices such as photo-detectors and/or solar cells. Current ways to introduce halide anions as ligands on surfaces of NCs produced by the hot injection method are based on post-synthetic treatments. In this work we explore the possibility to introduce Cl in the NC ligand shell in situ during the NCs synthesis. With this aim, the effect of 1,2-dichloroethane (DCE) in the synthesis of CdSe rod-like NCs produced under different Cd/Se precursor molar ratios has been studied. We report a double role of DCE depending on the Cd/Se precursor molar ratio (either under excess of cadmium or selenium precursor). According to mass spectrometry (ESI-TOF) and nuclear magnetic resonance ((1)H NMR), under excess of Se precursor (Se dissolved in trioctylphosphine, TOP) conditions at 265 °C ethane-1,2-diylbis(trioctylphosphonium)dichloride is released as a product of the reaction between DCE and TOP. According to XPS studies chlorine gets incorporated into the CdSe ligand shell, promoting re-shaping of rod-like NCs into pyramidal ones. In contrast, under excess Cd precursor (CdO) conditions, DCE reacts with the Cd complex releasing chlorine-containing non-active species which do not trigger NCs re-shaping. The amount of chlorine incorporated into the ligand shell can thus be controlled by properly tuning the Cd/Se precursor molar ratio.